1. Field of the Invention
The present invention relates to polymeric microparticles and to non-aqueous dispersions of polymeric microparticles, particularly polymeric microparticles containing urethane linkages and to their use in coating compositions.
2. Brief Description of the Prior Art
Microgels are polymeric particles of colloidal dimensions, about 0.001 to 10 microns in diameter, which are prepared by dispersion polymerization techniques. Microgels are used in coating compositions where they improve the rheology of the coating composition and the properties of the resultant cured coating. Specifically, microgel improves the spraying efficiency of the coating composition enabling complete coverage in fewer passes of the spray gun. Microgel also improves the sag resistance of the coating and the metallic pigment pattern control of the coating composition using metallic pigments such as aluminum flake.
As has been mentioned above, microgels and their method of preparation are known in the art. British Pat. No. 967,051 and U.S. Pat. No. 4,290,932 disclose processes for preparing microgel in which a mixture of vinyl monomers, at least one of which contains at least two ethylenically unsaturated double bonds, is polymerized via aqueous emulsion polymerization techniques to form a crosslinked polymer in microparticle form.
U.S. Pat. No. 3,317,635 discloses non-aqueous dispersions of polymers which are prepared by polymerizing in an organic liquid, ethylenically unsaturated monomers which are soluble in the organic liquid but in which the resulting polymer is insoluble. Polymerization is conducted in the presence of a polymeric stabilizer which contains a segment solvated by the organic liquid and an unsaturated group attached to the polymer chain by a primary chemical bond. An example of this primary chemical bond would be the reaction of a hydroxyl group in the polymeric chain and an NCO group associated with the unsaturated group. For example, the stabilizer could be a copolymer of tertiary-butyl methacrylate and ethylene glycol monomethacrylate. This would then be reacted with allyl isocyanate. The stabilizer is then used in the polymerization reaction as described above. There is no disclosure in the reference of reacting NCO-prepolymers with an active hydrogen-containing material to form dispersions of polymeric microparticles.
U.S. Pat. No. 3,717,605 is similar to U.S. Pat. No. 3,317,635 mentioned above in that it also relates to non-aqueous dispersion of polymers in which ethylenically unsaturated monomers are polymerized in an organic liquid in the presence of polymeric stabilizer. The stabilizer is formed from reacting compound A with compound B, with compound A being solvatable by the organic liquid and compound B not being solvatable by the organic liquid and having a different polarity from compound A, and which contains a group capable of condensation reaction with compound A. The co-reactive groups in compounds A and B can be those which form urethane linkages such as isocyanate and hydroxyl groups. Once again, there is no disclosure in the reference of reacting a preformed NCO-prepolymer with active hydrogen-containing materials to form dispersions of polymeric microparticles.
U.S. Pat. No. 3,748,294 relates to polyurethane microgels which are prepared by reacting a polyisocyanate and a polyhydroxy precursor wherein the reaction is carried out in an inert organic liquid in which the polyisocyanate and polyhydroxy precursor are essentially soluble. An example of a polyisocyanate would be a compound and optionally a polymeric compound having attached thereto two or more reactable isocyanate groups. Examples are 2,4-toluene diisocyanate and condensates thereof comprising 3 or more isocyanate groups. An example of the polyhydroxy precursor would be condensates of various fatty acid esters of polyols such as pentaerythritol, for example, pentaerythritol-12-hydroxystearic acid condensates. The polyisocyanate and the polyhydroxy precursor are polymerized under conditions to form the crosslinked polymer which is dispersed in particulate form in the organic liquid. There is no disclosure in the reference, however, of reacting in dispersed form a preformed NCO-containing prepolymer which contains a segment solvated by the organic liquid with an active hydrogen-containing material to form dispersed polymeric microparticles.
In the JOURNAL OR CELLULAR PLASTICS, January/February 1981, reference is made to PHD polyols, an abbreviation of the German "polyharnstoff dispersion". PHD polyols consist of particles of polyurea dispersed in a polyether polyol. These dispersions are not prepared with a preformed NCO-prepolymer and are not dispersed in a liquid hydrocarbon.
U.S. Pat. No. 4,425,468 deals with poly(urea-urethane) dispersions. In this patent, a polyisocyanate is reacted with a polyfunctional amine in the presence of a polyol free of ethylenic unsaturation. Reaction is conducted in the presence of a diluent such as an ethylenically unsaturated compound such as methyl methacrylate, and an active hydrogen-containing polymerizable ethylenically unsaturated compound such as hydroxyethyl methacrylate. A polymerizable ethylenically unsaturated urea-urethane polymer dispersed in a polymerizable ethylenically unsaturated compound is formed. There is no disclosure in the reference of reacting a preformed NCO-containing prepolymer with an active hydrogen-containing compound having a functionality greater than or equal to 2, nor of forming dispersions in liquid hydrocarbons.
U.S. Pat. No. 4,147,688 discloses a method for preparing acrylic microgels and the use of these microgels in coating compositions. Although the acrylic microgels provide excellent sag resistance with acrylic coating compositions, they are not as effective in this respect with compositions derived from condensation polymers such as polyesters and polyurethanes. In accordance with the present invention, it has been found that a certain class of polyurethane microparticles provide excellent sag resistance for coating compositions based on condensation polymers such as polyesters and polyurethanes.